What is the input-tolerant function?
A CMOS logic IC with an input-tolerant function allows a voltage of up to the maximum operating voltage to be applied to an input while the power supply is active or at 0 V. The input-tolerant function allows level shifting from a higher voltage to a lower voltage.
Examples of level shifting
Down translation from 5 V to 3 V: Use the 74VHC or 74LCX Series.
Down translation from 3 V to 1.2 V: Use the 74VCX Series.
The following shows an equivalent circuit for a typical CMOS logic IC.
The diodes on the input side are inserted for the purpose of ESD protection whereas those on the output side are parasitic diodes.
The diode between the input and the power supply might turn on if a voltage higher than VCC is applied or voltage is applied when the IC is off. In this case, the IC might be destroyed by the resulting large current. Device destruction can be prevented by using an IC with an input-tolerant function, i.e., an IC without a diode between the input and power supply.
It is easy to distinguish whether an IC has an input-tolerant function.
Let’s look at the datasheets for ICs of the TC74HC and 74VHC Series.
See the input voltage shown in the Operating Ranges table.
The datasheet of the TC74HC Series shows VIN＝０ to VCC. This means that the input pin is tolerant of only up to VCC. Therefore, the TC74HC Series does not have an input-tolerant function.
In contrast, the datasheet of the 74VHC Series shows VIN＝０to 5.5 V, meaning that up to 5.5 V can be applied to the input pin regardless of VCC. Therefore, the 74VHC Series has an input-tolerant function.